Flexible electronics, also referred to as flex assemblies, is a technology in which electronic devices may be mounted on flexible substrates. These devices mounted onto the flexible substrate may be referred to as surface mounted devices (SMDs). Examples of such substrates include but are not limited to polyimide or PEEK. The completed flexible electronic assembly may conform to a desired shape or may flex during use, such as may be desirable for tightly assembled electronic packages or where the assembly is required to flex during normal use. By way of example and not limitation, hearing aids or other hearing assistance devices may be fabricated using flexible electronics. Flexible electronics may be passive wiring structures used to interconnect integrated circuits and other electronic SMDs. Flexible electronics may be used to make interconnections to electronic assemblies directly or by use of connectors to a secondary electronic assembly.
A segmented flex assembly may include more than one flex circuit segments. These flex circuit segments, which may be referred to as flex circuit boards, may be connected using board-to-board connectors. Board-to-board connectors may be surface mounted on to flexible circuits. Elements of a board-to-board connector may include a board-to-board connector plug and a complementary board-to-board connector receptacle. It is desirable to increase the robustness of these board-to-board connectors.
Given the flexibility of the substrate on which the board-to-board connectors are attached, reliability issues may arise without a stiffener to support the flexible circuit in the location of the board-to-board connectors. For example, the lack of a stiffener may lead to bent or broken connectors during assembly and repair, and may lead to circuit delamination during the detaching process, with the connector tearing apart from the flex. A stiffener may be fabricated using a rigid material, such as polyethylene terephthalate (PET), FR4 board or stainless steel. PET is a thermoplastic polymer resin, and an FR4 board is a glass epoxy which has been used to fabricate printed circuit boards (PCBs). Generally the stiffener is either built into the board during the fabrication process or attached during subsequent assembly steps using an adhesive. Building the stiffener into the board during the fabrication process greatly increases the price of the flexible circuit assembly. Fabricating the stiffener into the board also reduces the circuit board's versatility, as the presence of the stiffener on one side limits the locations connectors can be attached and the resulting possible assembly orientations of the flexible circuit. A stiffener may be attached post assembly, but the additional assembly steps require additional time and materials. For example, common methods involve the use of either a thermoset or pressure sensitive adhesive. These methods are generally complicated, with the adhesives and stiffener boards supplied separately and combination completed in house by assembly operators.
Underfilling may involve a process in which epoxy or other adhesives are applied under SMD(s) mounted on the flexible substrate, and then cured to harden. However, underfill cannot be effectively used to further reinforce the connector without the need for a stiffener, as underfilling can wick into the connectors, preventing them from making electrical connections and functioning properly.
Accordingly, there is a need in the art for improving flexible circuit assemblies, including improving connections between boards of the flexible circuit assemblies.